Immunoassay-based techniques for identification and quantification of various analytes and specifically particle agglutination immunoassays are widely used in medicine, pharmacology and food industry. There are several ways to conduct particle agglutination assays and to detect positive agglutination reactions. Conventional agglutination tests are based on the formation of aggregates of colloidal particles in the presence of an analyte in the sample. Agglutinated heavy aggregates can be revealed by the naked eye or by expensive equipment, such as spectrophotometers, nephelometers, scanning laser microscopy, magnetic moment analysis, photon correlation spectroscopy and fluorescence analysis, and other instruments that measure transmitted, absorbed, or scattered light. Although, these instrumental methods have now been applied to a wide variety of commercial assays, they are not considered simple rapid tests because expensive instrumentation is required.
Particle agglutination assays can be performed either in a microtiter plate or on hydrophobic agglutination slides. The slide is usually kept rotated for several minutes and agglutination of latex particles is recorded visually while rotating the slide.
The detection of the agglutination reaction by the naked eye is a subjective procedure. Particle agglutination results vary from one clinical laboratory to another and reported results obtained with the same serum may vary even between technicians in the same laboratory. This situation makes it difficult for the physician to interpret reported results. Indeed, in some medical centers, physicians will request that the same technician perform all HIV antibody testing and blood typing to maintain consistency. This poses problems for the physician when the technician leaves or the physician moves to a new hospital or to another city. In other laboratories, several technicians interpret the same results independently. On the whole, because of the subjective interpretation of the results, the particle agglutination assays may suffer from higher numbers of false positives and false negatives. This situation affects significantly the performance (clinical sensitivity and specificity) of the assay.
There are mainly three serious drawbacks of the conventional particle agglutination methods: 1) a long analysis time; hence, the need for mechanical rotational motion of microtiter plates or agglutination slides to accelerate the agglutination process, 2) a limited analytical sensitivity of the assay, and 3) difficulty and subjectivity in interpretation of the assay results. In order to circumvent these drawbacks of agglutination assays, a number of approaches have been developed.
In particular, it has been shown that the detection rate and sensitivity of coated particle agglutination immunoassays are increased in ultrasonic standing waves (Wiklund M, Hertz H M. Ultrasonic enhancement of bead-based bioaffinity assays. Lab Chip. 2006 October; 6(10):1279-92). U.S. Pat. Nos. 5,665,605 and 5,912,182 issued to Coakley et al. and incorporated herein by reference in their entirety, disclose a particle agglutination method based on the use of ultrasound to enhance the agglutination process; specifically, to shorten the time of analysis and to increase the sensitivity of the assay.
As known in the prior art, when particles suspended in a fluid are subjected to an ultrasonic standing wave field of a particular single frequency, the particles displace to the locations of the standing wave nodes. The concentration of particles in the standing wave field increases the rate of particle agglutination and the sensitivity of antigen detection. The rate is increased because the local concentration of beads is accentuated in a standing wave field.
As a result of applying a standing wave field to the resonator cell, microparticles are clumped together in one of two ways. In the first, specifically-bound agglutinated microparticles form a strong immunochemical or another specific bond in an aggregate. The second type of aggregates is in the form of clumps of nonspecifically-bound microparticles that are attached to each other by various weak forces.
Despite the significant improvement of the particle agglutination tests due to application of ultrasonic standing waves, this approach suffers from the same drawback as other particle agglutination-based methods: significant error due to the presence of both specifically-bound and nonspecifically-bound aggregates. A solution to this problem could be provided by efficient microstirring capable of destroying these nonspecifically-bound aggregates.
Stirring liquids is a necessary part of many industrial, chemical and pharmaceutical processes, and there are many conventional stirring methods developed for these industrial processes. Since most of these processes are carried out on macroscopic scales, the stirring methods with various conventional mechanically or magnetically driven stirring elements are not applicable to small volume samples used in agglutination immunoassays. It has only been in the recent years that stirring of small quantities of liquids has become technologically relevant in the context of microfluidics, since stirring and mixing are often crucial to the effective functioning of devices manipulating with small quantities of liquids. (Nguyen, N. & Werely, S. 2002 Fundamentals and applications of microfluidics. Boston, Mass.).
Numerous methods of stirring in microvolumes of liquid have been developed. These methods can be categorized to be of two types: with and without moving parts. The moving parts stirrers include microscopic stirrer bars, piezoelectric membranes or oscillating gas bubbles. The mixing can also be achieved without moving parts by action of electrical or acoustic fields on the liquid. U.S. Pat. No. 7,081,189 issued to Squires et al. discloses microfluidic stirrer and mixer driven by induced-charge electro-osmosis applied to electrolyte fluids. Liu et al. developed an approach to microstirring based on acoustic microstreaming around an array of small air bubbles resting at the bottom of the test chamber (Liu, R., Lenigk, R., Druyor-Sanchez, R. L., Yang, J. & Grodzinski, P. 2003 Hybridization enhancement using cavitation microstreaming. Analyt. Chem. 75, 1911-1917). When bubbles are vibrated by a sound field, they create steady circular microflows around them. U.S. Pat. No. 6,244,738 issued to Yasuda et al. discloses ultrasonic vibrators arranged in the stirring tube where several sample solutions are stirred and mixed by an acoustic streaming induced by ultrasonic vibration.
Applicability of these microstirring methods to ultrasound-assisted particle agglutination tests for destroying non-specifically bound aggregates and increasing the sensitivity of the tests is highly limited. It is difficult to use known stirring techniques without significant modification of the particle agglutination technology. Thus, there is a need for a method and device that would not only preserve all advantages of ultrasound standing wave-assisted particle agglutination technology but also provide efficient microstirring aimed to destroy nonspecifically-bound aggregates, therefore improving signal-to-noise ratio in quantitative assessment of the amount of immunochemically-bound aggregates.